1. Field of the Invention
The invention relates to a method of producing, using an isothermal forging process, circular metal articles reinforced internally by fibres over one or more selected parts of their cross-section. The invention also relates to isothermal forging apparatus, and to apparatus for use in making reinforcing fibre preforms which are used in the method.
The circular articles produced by the method are intended for use, inter alia, in the production of high strength-to-weight ratio rotors, such as the rotors of turbomachines for aircraft.
2. Summary of the Prior Art
The rotors of turbomachines are generally symmetrical about their rotational axis and have on their periphery or their side a plurality of blades serving as either compressor blades or turbine blades. These rotors usually rotate at high speed and experience severe stresses, due mainly to centrifugal force but also to vibrations of the machine and the accidental intake of foreign bodies. The design of such rotors is a compromise between aerodynamic or hydrodynamic performance, strength and weight.
In endeavours to improve the strength-to-weight ratio it is known to reinforce the rotors by rings or bands made of stronger materials and having a higher modulus of elasticity, also known as Young's modulus.
Rotor reinforcing rings are known which have a fibrous structure, particularly of fibres of silicon carbide, boron carbide or other high-strength material, the fibres being embedded in a metal matrix. French patent No. 2607071 describes one example in which silicon carbide (SIC) fibres are assembled as a helical ribbon or preform, and another ribbon made of the material of the matrix is interleaved between the turns of the silicon carbide fibre ribbon. The assembly is pressed at high temperature, and the pressure forces diffuse the matrix material between the fibres to give cohesion to the assembly.
In another embodiment preforms are used which are each formed by a single fibre wound in a flat spiral. This ensures excellent regularity of the radius of curvature of the fibre for enhanced strength of the ring.
The fibre can be held in position before pressing by radial strips or filaments extending alternately below and above the fibres to be held, as in the case of the yarns of a woven fabric. This solution is difficult to carry out and has the further disadvantage of making the fibres wavy and thus reducing their ability to withstand a tensile force.
To avoid this drawback European patent No. 0490629 proposes forming a spiral groove in a strip of the same material as the matrix, placing the fibre in the groove and holding the fibre in position by spraying on an organic binder or a plasma of the matrix metal to provide a preform containing both fibre and matrix metal. This solution does not have the disadvantage of making the fibres wavy, but is expensive and difficult to carry into effect because the grooves into which the fibre has to be introduced are very narrow. The ring is then produced by hot compression of the preform at a high enough temperature and pressure for the metal to penetrate between the fibres and be welded around them. If an organic binder is used, this is destroyed by pyrolysis during the temperature rise, leaving only minimal carbon residues.
Rotors can be reinforced with such rings by mechanical assembly of the ring on the rotor, in which assembly there are bound to be gaps. The ring therefore becomes effective only when the rotor is sufficiently deformed to close the gaps.
The rings can also be banded around the rotors. This arrangement provides excellent cohesion between the ring and the rotor but has the disadvantage of subjecting the ring to tensile prestressing, which reduces its ability to retain the rotor. Also, the rotor geometry must be adapted to receive the ring.
Reinforcing rings can also be slid onto the rotor periphery or introduced into grooves machined in the rotors, then brazed or welded thereto by any means. This solution provides excellent cohesion between the ring and the remainder of the rotor, but has the disadvantage of subjecting the fibres to two heat cycles, so that their degradation is increased. For example, silicon carbide (SIC), which is often used for making reinforcing fibres, may react with the metal of the matrix at high temperature. To reduce this reaction the fibres may be covered by a carbon layer, but this diffuses into the matrix metal and alters the metallurgical structure of the matrix. The reaction of the fibre material with the matrix is therefore merely delayed.
The advanced process of isothermal forging is also known wherein the article is heated in a vacuum chamber or in a controlled atmosphere to a temperature at which the metal can be deformed superplastically, the article is placed in a former heated to the same temperature, and a continuous pressure is applied to the article. This process leads to an outstanding and very homogeneous metallurgical structure of the article. It also enables the article blank to be made in a number of parts which are welded together during the forging.